Previously, we characterized mouse and human cDNA clones encoding a new transmembrane EGF-like protein. The predicted protein from this cDNA (termed delta like or dlk) is highly homologous to invertebrate homeotic transmembrane proteins including Notch and Delta of Drosophila, also containing EGF-like repeats at the extracellular domains that intervene in protein-protein interactions between cells. These interactions generate signal transduction events which are involved in differentiation decisions. Our interest is to understand the function of Notch in immune system cells and tumors, and in other normal and neoplastic tissues. Notch is an extensively studied gene in Drosophila, which intervenes in differentiation decisions during the fly's development. The function of a highly conserved and highly expressed homolog of this gene in the immune system of adult mammals is unknown, but some evidence suggests that it may have a role in the origin of T-All. Our objectives for the year were to continue with the functional studies on the gene, and to express the Notch protein in bacteria to obtain antibodies against the protein. Partial Notch protein sequences have been expressed in E. coli and purified. Production of rabbit antisera to extracellular and inracellular domain is ongoing. RT-PCR and immunohistochemistry analysis have demonstrated the presence of mRNA in tissues. The functional role of ankyrin repeats in modulation of DNA regulatory elements has been demonstrated.